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However, in coils with opposite current directions, the magnetic flux lines are separated and driven away from the center of the coils. Therefore , excluding the magnetic field substantially decreases induction heating efficiency and temperature uniformity.
2.3. Magnetic concentrator
A magnetic concentrator has material properties resembling those of a transformer coil, i.e., high conductivity and low resistance. It is used in the induction heating process for local enhancement of the magnetic field, which thus improves the heating effect. When the processed workpiece is close to the induction coil, the proximity effect changes the current distribution. The current is distributed along the coil surface, which causes a loose distribution of eddy current on the processed workpiece. Thus, the surface heating effect is in sufficient. Use of a magnetic concentrator can enforce the coil current and eddy current distribution, which then enhances its heating rate[23] .
This study proposes a novel magnetic shielding induction heating method to solve the repulsive proximity problem. By using ferrite mate-rials to separate the conflict magnetic fields to eliminate the influences of repulsive proximity effect, the heating efficiency and temperature uniformity were thus enhanced. Fig. 2 shows the magnetic flux fields of two adjacent opposite current coils combined with a magnetic concentrator and magnetic shielding material. Whereas the magnetic concentrator can separate the magnetic fluxes between the magnetic concentrators and concentrate the magnetic flux below the coils, the magnetic concentrator cannot avoid the proximity effect that occurs under the coils. Thus, the center of the workpiece has a lower heating efficiency and a lower temperature . In contrast with the magnetic concentrator method , the proposed magnetic shielding method completely separates the magnetic flux fields from different coils and drives the magnetic flux uniformly throughout the workpiece surface.This study performed a series of induction heating experiments in three typical single -layer coils: the reciprocated single -layer coil, the single-layer spiral coil, and the rectangular frame coil . Both heating efficiency and temperature uniformity were compared.
A series of induction heating experiments were performed to evaluate the use of ferrite materials in the proposed magnetic shielding induction heating method . A normal mold steel AISI-P20 with dimensions of 170 mm long×170mm wide×20 mm thick was heated by a high frequency induced heating machine (H P-25KW, Hon or,Taiwan) .Fig. 3 shows the geometry and cooling channel ( diameter:8 mm) dimensions of the heated plate. The mold temperature controller was used to pass 60°C heat-transfer oil through the cooling channels during all heating/ cooling process. A thermometer and infrared ray thermal imaging system ( ThermoVision A2 0 , FLIR , USA) were also used to record the temperature variation of the heated-plate surface.
This experiment used three typical single-layer coils, the reciprocated single-layer coil, the single-layer spiral coil , and the rectangular frame coil, to perform a series of induction heating experiments. Fig. 4 shows the coil designs and heating area (shaded area). The coils were composed of 5 mm diameter copper tube , and the heating distance from the heated-plate surface to the coil was 15 mm in all three coils. Fig. 4( a ) shows that the reciprocated single-layer coil has a 2 5 mm pitch and a heating area of 75 mm × 75 mm. Fig. 4(b) shows that the single-layer spiral coil has a 15 mm pitch and a heating area of 75 mm×75 mm.Fig. 4(c) shows the rectangular frame coil around a 121 mm long × 76 mm wide rectangular area . The heating area was imitated with the front cover of a 5-inch smartphone.
In the magnetic shielding induction heating method, ferrite m ate-rials are used to separate the conflicting magnetic fields. The normal ferrite materials mainly have two types, the Mn–Zn and the Ni–Zn ferrites. Both ferrite materials are widely used in transformer coils, and are easily magnetized and demagnetized. Table 1shows the magnetic properties of the Mn–Zn and Ni–Zn ferrites. The Ni–Zn ferrite was selected for this experiment. The N i–Zn ferrite has much higher resistivity(108 Ω-m) compared to the Mn–Zn ferrite (8 Ω-m). The higher resistivity is advantageous to avoid the ferrite to be heated during the induction heating process, and therefore has less extra energy consumption.